Users of various forms of communications are constantly in need of increased channel capacity and bandwidth so that more information can be transmitted between nodes. Maps, imagery, voice, e-mail and other types of information now need to be transmitted between vehicles, satellites, and base stations, both fixed and airborne. At the same time, security concerns require encryption of the certain information, which exacerbates the problem of channel capacity in communication systems.
In its simplest form, the channel capacity of a communications system may be given as:
C=B log2(1+SNR) 
where B is the link bandwidth and SNR is the signal-to-noise ratio of the communications link. SNR can be expressed in terms of many of the driving parameters of the system as:   SNR  =            γ      ⁢              xe2x80x83            ⁢              P        t            ⁢              g        t            ⁢              g        r                    4      ⁢              xe2x80x83            ⁢              π        2            ⁢      N      
where Pt is the transmitter power, r is the distance between the transmitter and receiver, N is the noise power, and gt and gr are the antenna gains of the transmitter and receiver, respectively. The antenna gain can be related to the antenna aperture size via:   g  ≅            4      ⁢              xe2x80x83            ⁢      π      ⁢              xe2x80x83            ⁢      LW              λ      2      
where xcex is the wavelength of radiation and L,W are the length and widths of the (assumed rectangular) antenna.
Based on the above equations, there are multiple approaches to increasing channel capacity, including: 1) finding new degrees of freedom to be used in the communication channel, such as polarization, higher frequencies; 2) increasing bandwidth; and 3) increasing the signal-to-noise ratio of the current communication link.
Increasing the signal-to-noise ratio in a communications system can be addressed in a number of ways. Some approaches are more direct than others. Two simple methods of increasing the signal-to-noise ratio are; 1) to increase the transmitter signal power and/or 2) make the communication link physically shorter. In many instances, these approaches are not feasible.
Another approach is to increase the apparent aperture of the antenna system. This can be done physically whereby a larger antenna aperture is used. As with increasing the transmitter power, however, this approach may also not be feasible in many applications. In mobile and satellite communication systems, transmitters and receivers move relative to one another. This can be viewed as a drawback, or this relative movement can be used as an opportunity to incorporate some rather sophisticated signal processing to improve performance.
Broadly, this invention exploits the relative movement of a receiver and transmitter in a communications system by electronically synthesizing a larger apparent antenna aperture, thereby increasing signal-to-noise ratio.
A preferred embodiment is disclosed wherein the aperture is synthesized via pulse-to-pulse coherence in a cellular communications receiver moving relative to a fixed transmitter. However, the aperture may be synthesized more broadly through angular diversity due to motion, regardless of whether the transmitter is fixed and the user or vehicle is moving, or the user or vehicle is fixed and the transmitter is moving, as would be the case with an airborne transmitter or geosynchronous satellite. The invention is further applicable to the case where both the receiver and transmitter are both moving, as would be the case with a vehicle downloading from a satellite, for example.
Although in this example the aperture is synthesized via pulse-to-pulse coherence in a receiver moving relative to a fixed transmitter, the aperture may be synthesized more broadly through angular diversity due to motion, regardless of whether the transmitter is fixed and the user or vehicle is moving, or the user or vehicle is fixed and the transmitter is moving, as would be the case with an airborne transmitter or geosynchronous satellite.
In terms of operation, a method of improving the performance of a receiver according to invention would include the steps of determining the apparent angle between the receiver and transmitter relative to the direction of movement, using the apparent angle to produce time-delayed replicas of the received signaling stream, and coherently adding the time-delayed replicas of the signaling stream to synthesize the increased apparent receiver antenna aperture.
The apparent angle may be estimated in different ways. For example, a Doppler shift in the carrier frequency may be determined due to the relative motion, and the apparent angle may be found as a function of the Doppler shift. Alternatively, the apparent angle may be determined as a function of the time delay of the arrival of the signaling stream.
Since only the receiver is modified according to the invention, existing transmitters and infrastructures can be used without modification. Significant cost potentials can be realized via economy of scale, due to relatively simple FFT processing. Although some data buffering is required, only a few number of beams need to be synthesized, in contract to more complex military SAR configurations. Use of the inventive technology lowers antenna side lobes, resulting in a higher signal to noise ratio, which, in turn, may provide for better reception, more users at a given time, and enhanced services such as image/video capabilities, which are currently problematic to implement.